The invention refers to a process far plating of work pieces, whereby for the formulation of a functional, especially corrosion-resistant and wear-resistant metallic coating, and at least phosphorus and nickle-containing alloy is precipitated from an electrolyte. The invention further refers to an appropriate coating.
Metallic coatings, in addition to their decorative uses, serve above all to create a functional coating of work pieces in order to deal with application-related load mechanisms by suitable surface properties such as, for example, hardness, wear resistance, frictional behavior or thermal and chemical resistance. The generation of such protective coatings is usually by galvanic precipitation. Here, a difference must be made between electrolytic precipitation on the one hand, where electro-crystallization takes place and, on the other hand, electro-chemical precipitation without external current which comprises a simple immersion system without external current source and anodes. The metal can be precipitated by these methods both onto metallic and also onto non-metallic work piece surfaces. Due to high contour definition during preciptation, the external current-less precipitation is used especially in the case of work pieces having a complex geometry requiring very tight tolerances. In this case electrolytic precipitation is severely limited due to the distribution of local cathode current densities related to the geometry.
In order to achieve a corrosion and wear-resistant metallic coating it therefore is known to precipitate higher phosphorous-containing polyalloys with nickel as the base metal without external current. Experiments have shown that the admixture of 0.5 to 1.5% by weight of cobalt to a nickel-cobalt-phosphorus coating with a phosphorus content of between 10 and 13% by weight can contribute to an increase in the internal pressure stress and the measurable hardness. It is further known that the phosphorus contents in the alloy matrix has a material influence on the passivation properties of a metallic coating. For example, corrosion tests have shown that the improved corrosion resistance can be achieved with a phosphorus contents of 14 to 21% by weight. However, it a disadvantage of the external current-less precipitation that due to the absence of express reaction mechanisms such high phosphorus contents can not be achieved.
It is the purpose of the invention to describe a process for coating work pieces which makes possible increased wear resistance and hardness, as well as an improved anti-corrosive effect, with a materially increased ratio of elemental phosphorus. In addition, a suitable coating will be described.
This purpose is achieved with a process of a type described at the outset in such a manner that an at least quaternary alloy with the components nickel cobalt, tungsten and phosphorus is precipitated electrolytically as a coating.
Such a process makes possible a ratio of phosphorus in the metallic coating of between 14 and 21% by weight due to the electrolytic precipitation and consequent forced reduced reaction systematic at the border between electrolyte and work piece. The invention is further based on the surprising finding that with the cathodic joint precipitation of nickel, cobalt, tungsten and phosphorus an alloy coating is formed which is marked by a high degree of corrosion and wear resistance.
It is of special advantage, depending on the functional application of the coating, to cathodically precipitate an alloy containing at least another component, preferably tin, lead, molybdenum, rhenium or vanadium. In this manner, for example, sufficient temperature resistance, the ability to be soldered, magnetic permeability or a suitable friction coefficient can be achieved in accordance with applicable requirements.
In accordance with a preferred extension of the invention, non-metallic particles, preferably carbide or mixed carbide crystals are embedded in the metallic matrix of the coating in order to additionally increase corrosion and wear resistance and hardness. Here it is of advantage to utilize carbides of boron, silicon, tungsten, vanadium and/or titanium. It is further of advantage if particles of a grain size of 0.1 to 1.5 xcexcm are used.
Alternatively ultrafine particles with diameters in the nanometer range or particles with a grain size in excess of 1.5 xcexcm may be used, depending on the desired border layer properties. In accordance with a further characteristic of the invention particles are deposited in different concentration within the range of the thickness of the coating. In this manner it is possible, for example, to achieve a higher concentration of deposited particles in the area of the coating facing the base material and a lesser concentration in the surface area of the coating. Therefore, the microporosity of the coating at the location of the particle deposit can be adjusted as required.
In accordance with the advantage of another extension of the invention the particles are added to the electrolyte as a dispersed phase and embedded into the alloy deposit with the galvanic precipitation. This addition, above all, can be traced to adsorption, electrostatic attraction and mechanical inclusion. For this purpose, the particles are held in suspension in the electrolyte by agitation of the galvanic bath. In the generally known manner particles are held in suspension, for example, by stirring or by blowing air into the bath. In order to be able to change the microporosity of the coating it is further useful to affect the concentration of the particles in the alloy precipitation by changing the bath agitation. In addition, the invention proposes that additionally color pigments preferably of titanium dioxide, are embedded which ensure a high degree of resistance to light and to climatic conditions.
To achieve the above purpose the invention proposes, in addition, a corrosion and wear-resistance coating which is achievable especially by the above-described process and which is characterized by a composition of generally
0.5 to 2.0% by weight tungsten
1.0 to 2.0% by weight cobalt
15 to 20% by weight phosphorus and at least
10% by weight nickel
Finally, especially in order to further improve wear resistance, is proposed that 30 to 39% by volume of boron, carbide embedded in the metallic matrix of the coating.